The thyroid is a gland located in your neck, just beneath the Adam's apple. It releases hormones – thyroxine (T4) and triiodothyronine (T3) – that increase the body's metabolic rate.
A metabolic rate is the rate of chemical processes occurring within the body that are necessary to maintain life. In metabolism, some substances are broken down to provide energy for vital processes while other substances are made.
The thyroid gland is self-regulated by thyroid-stimulating hormone (TSH), made by the pituitary gland (sometimes called the master gland) in the brain. TSH stimulates the thyroid gland to produce thyroid hormones (T4 and T3). When thyroid hormone levels in the body are high, TSH production will "switch off," stopping the thyroid from making more T4 and T3.
Hyperthyroidism occurs when the thyroid gland becomes overactive, making an excessive amount of thyroid hormones. It is much less common than hypothyroidism (underactive thyroid gland) and has a variety of causes.
Hyperthyroidism is 5 to 10 times more likely to occur in women than men. Risk factors for developing hyperthyroidism include having another autoimmune disease (e.g., type 1 diabetes, pernicious anemia), a family history, eating large amounts of iodine, being a woman, and previous goiter.